Tyred Out Yet?
They’re the four pieces of rubber that are the most important part of your car as they are the only part of the vehicle that touches the road. Tyres, a criminally underrated part of your chosen chariot, are also responsible for confusion and angst. There’s numbers on the side and people hear about different…things about compounds. Here’s the skinny on what they’re all about.
The numbers: On the sidewall of a tyre will be information about the tyre, with the simplest being something looking like 
this: “225/60/17”. Tyres are, naturally, wrapped around wheels. The wheels will have a diameter and in a metric age are still listed by inches. Smaller cars will generally have wheel diameters of 14 to 16 inches, medium cars generally up to 17 whilst large cars can be up to 19 as standard and certain cars can be fitted with up to 22 inch diameter wheels. The tyres then will have that info and will be showing the diameter as the last number. The other two work hand in hand, with the biggest number being the width across the tread of the tyre in millimetres (225) and the sidewall height or profile, from the rim of the wheel to the tread, expressed as a percentage of the tread width. This means that our 225/60 tyres will have a sidewall being equalling 60 percent of the width of the tread.
Tossed into that set of numbers could be something like this: 225/60VR/17.
For over 40 years, tyres have been constructed in a radial design (http://en.wikipedia.org/wiki/Radial_tire) and have speed rating applied to them for certain intended purposes. A V rating permits that tyre to be run up to 240km/h with the R denoting a Radial construction. Although Australia, for example has a maximum permitted road speed of 110 km/h, there are places where a car can be travelling above that limit, such as a race track plus, in a world market, other countries have higher permissable speeds.
Rolling diameter: this is nothing more complicated than how to mix and match tyre and wheel sizes to give, effectively, the same size circle. Let’s use 205/65/15 tyres for a mid 1990s Commodore. The OVERALL diameter is about 647mm. The driver decides to fit some 19 inch diameter wheels; if they were to fit the same PROFILE (65) tyres the overall diameter would be well over 700mm. This is both currently illegal and in real and serious danger of the rubber fouling the inside of the wheel arches plus can give a false reading to the speedometer. Therefore a LOWER profile tyre needs to be used. By using 35 profile tyres (height is 35% of the width of the tyre) the overall diameter is brought back to 647mm. Of note is how a profile can affect the perceived ride of a car; a higher profile tyre will have more sidewall “give” than a lower; think of a well padded cushion versus a slim one of the same material. The tyre and wheel increase can also be known as “plus one/two etc”.
Compounds: rubber can be soft, it can be hard and anywhere in between. Followers of motorsport will be aware of hard/soft/medium compounds being bandied around as easy as we talk about the weather and really, that’s pretty much all this means. Soft tyre compounds will give more grip overall however may wear quicker due to the extra grip, whereas hard compound may not grip quite so well, will last longer and may not give a softer ride. However, there’s a little bit more to it such as where the tyres will be used as weather conditions on a ongoing basis.
Tyre pressures: these can be read as PSI (pounds per square inch) or kilopascals.
Most companies use PSI and it’s and indication of how much pressure is required to stop excessive wear of the tyre balanced against the expected load the tyres will carry (car, passengers and cargo). This information for each car can be found on a placard attached to the car and also on the tyre sidewall. It’s important to have correctly pressure tyres to stop wear either on the centre of the tyre (over inflated) or on the edges (underinflated) plus ride quality and handling can be severely affected
Run flat/spacesaver tyres: run flats are intended to give some measure of sidewall support should the tyre have deflated past its normal recommended pressure for any reason and will allow the vehicle to be driven, to a point, where the tyre can be replaced. Comfort levels are not included as part of the design. Space savers are becoming more and more common, partly to save weight but primarily 
to, as the name implies, save space, generally in the boot of the car. They are NOT intended to be used for anything other than to temporarily replace a normal tyre and are an emergency item.
Brands: there are heaps to choose from, such as Bridgestone, Dunlop, Kumho, Firestone, just to name a few; the actual construction quality will be of a higher standard in the better known brands and independent testing tends to show better overall grip and performance levels.
Tread patterns: there’s a couple of terms, such as asymmetric and directional, that catch people. Most “normal” tyres are symmetrical, in that the tread looks the same on the left and right of the tyre and are generally for everyday use. Asymmetrical tyres, generally for performance applications, will have an unbalanced look, for example with an unbroken groove or two one one side and a broken up tread pattern on the other. Directional tyres are akin to an arrowhead patter, with grooves leading from the outside to the centre at an angle and intended to give 
maximum water dispersion.
Any reputable tyre fitment centre such as Bob Jane’s or Jax can assist in explaining these in more detail. The humble tyre, more important than you think.
Level 2, 9 Help Street
Bill says:
Tires can make a real difference to your car. After 20,000 Km my new Volvo C70 started to make a loud rumbling noise which made me think a wheel bearing was defective. I took the car to the dealer who checked the bearings and said they were fine. He suggested the noise was due to tire wear, although a visual inspection showed the tires were at most 50% worn and the wear was even with no obvious bald patches or one side more worn than the other. I continued to drive the car and the noise continued to get louder, plus the car’s normal driving characteristics also subtly changed. The steering on this nice new car became a nightmare. The car just wanted to change direction all the time randomly (it did not pull in any one direction). Took it have the front end aligned and was told it was OK and needed no work. Eventually at 39,000 Km I bit the bullet and got the tires replaced. The tire fitter was curious why I wanted to replace tires that had more than 10,000 Km of tread left on them and looked to be in perfectly serviceable condition. My response must have shocked him as I said if the new tires did not improve the feel of the car and eliminate the noise which by now was obtrusive at any speed I would have to change the car. To my great relief the new tires cured both the noise and steering problems and my car drives like new again.
November 28th, 2013 at 3:59 pm
Barry Elfverson says:
Re Tyred Out Yet, I would like to submit the following comments regarding Run flat/ Spacesaver tyres:
Spacesaver tyres, in my opinion, are one of the most irresponsible and criminally negligent items being included in today’s modern hi-tech motor vehicles (including in-car radio/phone/apps computers and gismos as an aside).
For Spacesaver wheels/tyres the reasons for this are they –
1. Do not comply with the correct tyre size for the given vehicle;
2. Do not comply with the ADR requirements that only same size wheels/tyres are fitted across the same axles ( front or rear);
3. Do not comply with the minimum speed/load for the given vehicle – small, large, standard-powered or high/turbo-powered. How can an 80kmh wheel/tyre (maximum)be fitted to a vehicle with a speed capability exceeding 200kmh on today’s roads??
4. The manufacturers, dealers, and road authorities do NOT CLEARLY advise that these wheels/tyres should NOT be used for long distance front axle use. The steering of a vehicle fitted with these wheels/tyres would be severely compromised and lead to catastrophic results in inexperienced (and experienced) drivers hands.
5. Given our vast country, road and weather conditions, distances between towns/cities, and the inattention or misunderstanding of some drivers, the maximum speed rating for these wheels/tyres – 80kmh – is VERY or MOST LIKELY to be EXCEEDED by such drivers. This could have very serious consequences such as accidents causiung deaths or serious injuries. This could well occur on distant or isolated roads and locations.
6. Given that they are fitted in many vehicles with Alloy wheels and expensive high-performance tyres, these spacesavers are a cost cutting exercise only – nothing more.
In many country areas, local garages and tyre dealers do NOT carry the range of high performance and unusual tyres and wheels – e.g. rim diameters, profiles, etc – which means either delays in travel or drivers opting to continue greater distances on these ‘useless’ wheels/tyres. Also, due to the high prices and outlays for stocking many modern tyres and wheels, many country dealers do not carry the range needed for the average traveller.
As for run flat tyres – again, this is a big country and, as stated above, replacement tyres are not always available everywhere. They only have a limited distance before they are useless as well.
This is Australia, not a small European country. Only full size spare wheels/tyres as fitted to a vehicle’s axles should be supplied with every vehicle.
Who is going to take responsibility when someone gets killed or seriously injured (if not already)? The Governments, road/traffic authorities, motor dealers, the vehicle’s drivers? What about the families and other persons travelling in these vehicles – what is their legal stance in all this?
Please, get rid of Spacesaver wheels/tyres forever. Run flat tyres also have limited life when being driven deflated in this vast country.
Barry
November 30th, 2013 at 10:09 am
Max says:
We had the same problem with a Volkswagen Jetta. It appears that the rear camber and toe out cause scalloping on the inside edge of the tyre which will then create the ruckus you experienced. I had a four wheel alignment done but noticed that the geometry remained the same and didn’t actually change very much based on a very simple spirit level and eyeball from the rear to front wheels. I have begun rotating the tyres every 5000km and the tyres are not scalloping and the tyre wear front to back is more even than before where the fronts wore much quicker that the rears due to the extra weight up front.
December 2nd, 2013 at 10:25 am